The principle of selective photothermolysis underlies many laser therapies and is used to treat such diverse dermatological problems as leg veins, portwine stain birthmarks, other ectatic vascular lesions, and pigmented lesions including tattoos. The dermal and epidermal layers containing the targeted structures are irradiated with light, usually from lasers or flashlamps. The wavelength or color of this light is chosen so that its energy will be preferentially or selectively absorbed in the structures. This leads to the localized heating with the intent of raising the temperature to a point at which constituent proteins will denature or pigment particles will disperse.
The pulse duration of the irradiating light is also important for selectivity. If the pulse duration is too long, heat absorbed by the structures will diffuse out into the surrounding tissues and will not be selectively heated to the degree necessary. If the pulse durations are too short, however, the light absorbing chemical species such as blood hemoglobin or tattoo dye particle will be heated too quickly causing vaporization. Theory dictates that the proper pulse width should match the thermal diffusion time of the targeted structures. For smaller vessels contained in portwine stain birthmarks, for example, these thermal diffusion times can be on the order of hundreds of microseconds (xcexcsec) to several milliseconds (msec). Larger leg veins have thermal diffusion times in the 5 to 100 msec range. Pigmented lesion particles can have diffusion times as short as nanoseconds (nsec).
Various types of lasers have been tested for selective photothermolysis in dermatological specimens. Q-switched alexandrite lasers have been successfully used to treat naturally occurring dermatological pigmentations and also tattoos. Long-pulsed ruby lasers have been proposed for the removal of hair. Nd:YAG lasers (operating at 1060 nm), carbon dioxide (operating at 10.6 micrometers), and argon (operating in the 488-514 nm range) have been suggested for the treatment of ectatic vessels. The most successful vascular treatments have been achieved using dye lasers, and specifically flashlamp-excited pulse dye lasers. These lasers operate in the 577-585 nm range where there are absorption band peaks for hemoglobin and also operate well in the pulsed mode that provides for good selectivity. With the proper selection of color and pulse duration, success rates of higher than 50% are common when treating smaller vessels. Unfortunately, dye lasers are limited in pulse durations to less than 1.5 milliseconds. Thus, they tend to be inappropriate for the treatment of larger structures that would require pulse durations of hundreds of milliseconds, at least according to the principle. Attempts are being made to solve this problem. Frequency doubling Nd:YAG has been proposed as a technique to generate long pulses at 532 nm.
The present invention is directed to a long pulse alexandrite laser for treating dermatological specimens. The use of alexandrite allows operation in and about the near-infrared, specifically in a 100 nm range surrounding 760 nm where alexandrite is tunable, and ideally at approximately 755 nm and a surrounding 50 nm range xc2x125 nm. Radiation in this wavelength range penetrates well while still achieving an acceptable ratio of hemoglobin to melanin absorption. Moreover, the use of a long pulse alexandrite laser, in contrast to short-pulse, Q-switched versions of the laser typically used on pigmented lesions and tattoos, yields two advantages: 1) the pulse duration now can match the thermal relaxation times of larger dermatological structures; and 2) the removal of the Q-switching element makes a laser system that is less temperamental and easier to operate.
Ideally, the laser generates a laser light output pulse having a duration between 5 and 100 msec, with an output up to 50 Joules and with a delivered fluence, Joules per square centimeter (J/cm2), between 10 and 50 J/cm2. Spot sizes between from 0.1 to 10 cm2 are preferred for efficient coverage of the targeted area. A light delivery system is provided that transmits the laser light output pulse to dermatological targets of a patient.
In specific embodiments, the pulse is comprised of multiple resonant modes, which are supported by a hemispherical resonator configuration. Preferably, a radius of curvature of at least one of the resonator mirrors is shorter than a focal length of a thermal lens induced in the alexandrite gain media during generation of the laser light output pulse. This desensitizes the laser to this lens.
In other aspects of the embodiments, an active pulse forming network is used to drive at least one flashlamp to pump the alexandrite, the network allows pulse periodic heating to achieve the effect of longer pulse durations. These pulse periodic principles, however, may be generalized to other types of flashlamp excited lasers
In general, the pulse periodic heating techniques may also be applied to other types of flashlamp-excited lasers, such as dye and ruby, as a way of efficiently generating effectively long pulses of limited fluences as required in selective photothermolysis applications, for example. In addition, pulse periodic heating may also be employed in other solid-state lasers, as well as gas-discharge lasers. These techniques rely on the use of a series of laser light pulses with a limited duty cycle that have a total duration of the thermal relaxation time of the targeted structure, blood vessels for example. The total power of the pulses is that necessary to denature the targeted vessels. The pulse periodic heating technique efficiently uses the laser by reducing the energy absorbed by the gain media to get to the laser threshold. This energy does not contribute to laser action and is lost. Most commonly, pulse periodic heating is useful in dermatological applications for flashlamp-excited laser that require pulses of 10 msec and longer. Numerous other effective pulse durations may be achieved, including effective pulse durations greater than 0.1, 0.5, 5, or 50 msec.
The present invention is also directed to a long pulse alexandrite laser hair removal system. The use of an alexandrite in the present invention allows operation in the near-infrared, which provides good penetration to the hair root while still achieving an acceptable ratio of hemoglobin to melanin absorption.
In specific embodiments, it is desirable to use an index-matching application on the skin sections to be treated. This substance covers the epidermal layer to provide better coupling of the laser light into the skin.
In other aspects of the embodiments, a topical indicator is also preferably used on the skin. Skin irradiation in the near-infrared generally does not produce any characteristic skin color change as is found when using dye pulsed lasers, for example.
Thus, it is difficult to know exactly what portions of the skin have already been irradiated during a treatment session. The visual indicator is thermo- or photo-responsive or otherwise responsive to the laser light pulse to generate a visible change. This provides the operator with a record of those parts of the skin that have already been treated.
The skin is preferably treated with laser pulses of greater than a millisecond, preferably approximately 5 to 50 msec. Each pulse should contain a fluence of between 10 and 50 J/cm2. During each treatment session, each treated section of the skin is preferably irradiated with one such pulse, although multiple pulses could be used. Even so, permanent and complete laser removal may require three to four repeat treatment sessions, with weeks to months long dwell times between each session.
The present invention is also directed to a combined sclerotherapy and light treatment method for the cosmetic, i.e., non-therapeutic, treatment of unwanted veins. It is similar to flamplamp-excited pulse dye laser-sclerotherapy approaches from the prior art. Substantially increased success, in the range of 90-100%, however, has been achieved by implementing a dwell time of between 12 hours and 6 months between the light-based therapy and the sclerotherapy. Preferably, the light-based therapy is performed before the sclerotherapy. Success can be achieved by performing the sclerotherapy followed by the light-based therapy after the dwell time, however.
In specific embodiments, an alexandrite laser operating in the 755 nanometer range is a preferred light source, although flashlamp sources could also be used.
According to another aspect, the invention also features a kit for the treatment of unwanted blood vessels. It comprises a light source for irradiating the vessels with light adapted to initiate destruction of the vessels. A sclerosing agent, such as a hypertonic saline solution, is also needed for injection into the vessels. Instructions are desirably provided with the light source that suggest waiting for a dwell time between the irradiation of the vessels and sclerosing agent injection.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.